Optocouplers made of a combination sulfoselenide photoresistor and LED presently exist. They are used to couple electrical signals in analog circuits that process audio signals. They have special properties that make them useful in applications such as soft switching, signal compressors and signal limiters. They consist of a LED shining light onto a photoresistor inside a light-tight package. The photoresistor, generally made of a sulfoselenide compound, has the property that its electrical resistance varies with the amount of light incident on its surface. When the current driving the LED is changed, the amount of light that is emits also changes proportionally, and consequently also the electrical resistance of the photoresistor. The particular characteristics that make such optocouplers useful in analog audio circuits are their relatively slow response times, that enable them to smooth the signals of undesirable sharp components, and their low distortion, or low propensity to generate harmonics of the signals which they conduct.
The previously available optocouplers that comprise sulfoselenide resistors have been made in packages for mounting on through-hole circuit boards. This prevents their economical usage in volume-produced audio equipment, where electronic circuits are now assembled on surface-mount circuit boards, rather than through-hole circuit boards, as the former lend themselves to higher component density and lower fabrication costs. Therefore, the prior art has some disadvantages.